Gradients and flows in Jupiter's magnetosphere determined from the anisotropies of ~1 MeV protons

Abstract

For near-isotropic distributions of energetic ions, the field-perpendicular first order anisotropies which occur are due both to plasma flows transverse to B, and to field-perpendicular spatial gradients in the ion flux. A technique has been developed to separate these by examining the variation of the anisotropy with ion energy. This has been applied to data obtained by the Anisotropy Telescopes instrument on the Ulysses spacecraft during the Jupiter flyby in February 1992. Results are presented for a crossing through the equatorial plasma sheet on the prenoon inbound pass, and for an interval of transitions between the southern plasma sheet boundary and the outer magnetosphere region on the dusk outbound pass. The velocity data show that the flow in the outer plasma sheet was directed sunward both inbound and outbound at speeds 100-200 km s^-1, while that in the outer magnetosphere was directed antisunward at similar speeds. Flux gradient scale lengths peak at ~0.5 R_J within the prenoon current sheet, and are ~0.2 R_J on the outer surface of the dusk plasma sheet.